Urethane covered paper machine press rolls are known to have excellent abrasion resistance, vibration dampening and load bearing ability. However, the widespread acceptance of such rolls especially in the processing of wet webs in papermaking machinery, has been hampered by the loss of the bond between the urethane cover and the metal surface of the roll in a relatively short space of time. There are many known adhesives which will provide an excellent long lasting bond of urethane to metal if the assembly is maintained in a dry environment. It has been discovered that when urethane covered rolls are used in a wet environment, the initial excellent bond provided by these commercial adhesives disappear because moisture manifests itself at the urethane-metal bond line. It was originally thought that this loss of bond was due to the innate hydrolytic instability of urethane bondin9 adhesive systems. I have found that hydrolytic instability is an inconsequential factor in the loss of the bond.
I have discovered that temperature differentials between the urethane cover and the metal cause moisture to be formed at the bond line or interface between the metal and urethane. When the assembly is subjected to pressures such as exist in paper machine press roll assemblies receiving wet webs through the nips thereof, additional water is driven into the bond line or interface, inasmuch as the urethane cover is permeable to water. In any event, the temperature differential between the cover and the roll and the wet conditions under which the roll operates in press couples of papermaking machinery, develops moisture at the bond line or interface and the bond soon disintergrates, loosening the cover on the roll and rendering it unfit for any further use on the paper machine.
It would, therefore, be an important improvement in this art to maintain the bond between urethane roll covers and metal paper machine press rolls throughout a long wear life.